LA JOLLA, Calif.ŚSerotonin is one of the best-
known
chemicals in the human brain, mainly due to its role in the reward center. But
now some of serotonin's receptors are getting some attention as
well, thanks to
new findings on how certain serotonin receptors interact with and affect some
drugs.

The laboratory of TSRI Prof. Raymond Stevens, a senior
investigator for the new research, pioneered the development of techniques for
the
determination of the 3D atomic structures of cellular receptors,
particularly those that fall into the class of G protein-coupled receptors
(GPCRs).
The Stevens laboratory has used X-ray crystallography to determine the
structures of 10 of the most important GPCRs in the human body, including the
▀2
adrenergic receptor, the A2a adenosine receptor, HIV-related CXCR4 receptor,
the nociceptin receptor, S1P1 receptor, H1 histamine receptor and the D3
domapine receptor. These receptors affect issues such as pain mediation,
inflammatory disease, antihistamine medications and mood.

"Because G protein-coupled receptors are the targets of
nearly 50 percent of medicines, they are the focus of
several major National
Institutes of Health (NIH) initiatives," Jean
Chin of the NIH's National
Institute of General Medical
Sciences (NIGMS) said in a press release. The
NIGMS partly funded this work through the Protein Structure Initiative. "These
detailed molecular
structures of two serotonin receptor subfamilies bound to
antimigraines, antipsychotics, antidepressants or appetite suppressants will
help us
understand how normal cellular signaling is affect by these drugs, and
will offer a valuable framework for designing safer and more effective
medicines."

Serotonin, for its part, is linked not only to mood
regulation and the brain's
reward center, but also to consciousness and the
body's sleep/wake cycles, according to Daniel Wacker, lead author and TSRI
graduate student.

Bryan Roth, a professor of pharmacology at UNC and a
collaborator on both studies, noted in a press release that serotonin receptors
"also mediate a host of effects outside the brain, for example on blood
coagulation, smooth muscle contraction and heart valve growth."

In the first study, Chong Wang, co-lead author and a
graduate student in the Stevens lab, and colleagues
determined the atomic
structure of serotonin receptor subtype 5-HT1B, and produced the receptor while
it was bound with either ergotamine or
dihydroergotamine, anti-migraine drugs
that activate 5-HT1B receptors. A special fusion protein known as BRIL was used
to stabilize the structures and
line them up in a regular crystal ordering.
When x-ray crystallography was applied, it revealed the atomic structure of the
receptor with a main
binding pocket as well as a separate, extended binding
pocket.

Wacker and colleagues took a
similar approach in the second
study in determining the structure of the 5-HT2B receptor bound to ergotamine.
The 5-HT2B receptor is a target most drug
developers want to avoid due to the
off-target, and usually harmful, effects that can result. In 1997, fenfluramine
and dexfenfluramine, two weight-
loss drugs, were pulled from the U.S. market
after being linked to heart valve disease, which Roth's lab later discovered
was thanks to heart valve 5
-HT2B receptors.

Wacker says they were "absolutely not" expecting the results
their studies presented.

"We knew that particular drugs hit both receptors, but we had
no idea how similar their interactions were, because if you
were to model the receptors
based on other structures, you would have never gotten to the results we've
gotten," he explains. "I think the biggest
surprise for us was that the same compound
binds in a very similar manner to two different receptors, but the receptors, in key areas of their structure,
are very different. So exploiting these particular areas and improving
the drugs will hopefully lead to making safer and more selective medications.
"

The labs of Profs. Eric Xu and Hualiang Jiang at the
Shanghai Institute of Materia Medica, part of the Chinese Academy of Sciences,
then used the receptor structures to
simulate the bindings of various drugs,
demonstrating that anti-migraine drugs, known as triptans, should bind well to
the 5-HT1B receptors, but poorly
to 5-HT2B receptors, while fenfluramine's
active metabolite ought to bind tightly with the 5-HT2B receptor. Roth's lab
found that ergotamine and LSD,
an ergotamine-derived hallucinogen, favor
▀-arrestin signaling at the 5-HT2B receptor.

"I think the big
breakthrough of this, the big insight from
these studies, comes from providing selectivity information," says Wacker. "A
lot of drugs that were
designed to target specific serotonin receptors were found
to also target the serotonin receptor 5-HT2B. It was later found that activation of this
receptor causes valvular heart disease and pulmonary hypertension. So looking at these structures and figuring out a way of how to avoid binding and
activation the 5-HT2B receptor, or even to find drugs that would antagonize it, would be very beneficial."

The two ways to avoid triggering the 5-HT2B receptor, Wacker
notes, would be to either modify a drug so that it is incapable of binding with
the receptor, or to assign an antagonist for the receptor. Moving forward, he
adds that looking at more of the serotonergic receptor class is important,
as a
close relative of the 5-HT2B receptors, the 5HT-2C receptor, is a good target
for appetite-suppressing anti-obesity drugs. The knowledge of both
structures,
he says, could allow for safer medications that would avoid fenfluramine and
dexfenfluramine's harmful side effects.

The two studies are titled "Structural Basis for Molecular
Recognition at Serotonin Receptors" and "Structural
Features for Functional
Selectivity at Serotonin Receptors," and appeared in two papers on March 21 in
Science Express, the advance online version of Science.